Generally, fuel cells are power generation systems that convert chemical energy into electric energy by an electrochemical reaction between hydrogen and oxygen to produce electricity and water as a byproduct.
Such fuel cells are expected to substitute for internal combustion engines due to their excellent energy efficiency, and thus it is considered that the stable supply of hydrogen is essential for the use of fuel cells as an alternative energy source in the near future.
Methods for supplying hydrogen to fuel cells include methods of producing hydrogen by the electrolysis of water, and methods of producing hydrogen by reforming hydrogen-containing feeds with steam (steam reforming reaction).
Herein, many kinds of hydrocarbons including natural gas, LPG, naphtha, volatile oil and kerosene can be used as the feeds.
Also, the steam reforming reaction occurs in the presence of a catalyst at high temperature.
Thus, a catalyst is required in places in which the steam reforming reaction occurs, and heat required for the steam reforming reaction must be supplied.
In the prior art, a method was used in which heat required for the steam reforming reaction was generated in a separate place, and then the generated heat was transferred to a hydrogen generating apparatus. However, this method has problems in that it is inconvenient to use and difficult to manage.
Accordingly, in recent years, the inside of a hydrogen generating apparatus was divided into two sections by a tubular or plate-shaped metal, such that one section could be used as a steam reforming section (reforming section), and the other section could be used as a section for generating the heat required for steam reforming reactions (combustion section).
Namely, heat required for steam reforming reactions is transferred to the hydrogen generating apparatus by transferring heat generated in the combustion section to the reforming section (steam reforming section).
Herein, a catalyst in the reforming section is, in most cases, located in catalytic reaction tubes of a small diameter, and several catalytic reaction tubes are symmetrically placed around a heat source depending on the capacity and size of the hydrogen generating apparatus.
However, in this hydrogen generating apparatus according to the prior art, there is a high possibility for a feed to be unevenly distributed. If this uneven distribution of the feed occurs, a reactor tube supplied with a relatively small amount of the feed can undergo a rise in temperature corresponding to unabsorbed reaction heat, thus causing abnormal phenomena by high-temperature deformation. This will now be described in detail.
In a process of packing the reactors with catalysts formed of grains such as spherical or cylindrical pellets, the catalyst grains are randomly packed and, as a result, the difference in pressure between the front and back ends of the catalytic bed will necessarily differ between the catalytic reaction tubes.
The non-uniform pressure drop caused by the catalytic bed as described above causes different amounts of fluid to flow through the reactors.
The pressure drop in the catalyst-packed reactors is well known in the art to which the present invention pertains, for example, as shown in the Ergun equation.
The present invention aims to artificially cause a pressure drop which is several-fold greater than the pressure drop caused by the catalytic bed so as to minimize the influence of the pressure drop caused by the catalytic bed among the overall pressure drop across the reactors, thus preventing a non-uniform flow in each reactor tube.
A prior Japanese patent similar to the present invention shows an example in which an orifice having a diameter of 0.4 mm is provided as a pressure drop means in order to make uniform flow in each reactor tube of a hydrogen generating apparatus.